Seismic isolation as a way of protecting structures from the damaging motions of earthquakes is an idea that has been known for about a century. However it is only relatively recently that this concept has been put into practice. The recent history and design criteria for rubber-based isolators or bearings is seen in the book Earthquake-Resistant Design with Rubber by J. M. Kelly, Second Edition, Springer-Verlag London Limited, 1997, particularly in Chapter Five. The two basic types of isolation systems that have been employed are elastomeric bearings and sliders. The most widely adopted unit is the elastomeric bearing. Its most recent form is as a multilayered laminated bearing with alternating layers of rubber and steel. The stiff steel plates provide lateral constraint of each rubber layer when the bearing is subjected to vertical load, but does not constrain the horizontal shearing deformation of the rubber layers. This produces a bearing which is very stiff in the vertical direction and very flexible in the horizontal direction. Rubber energy-absorbing bearings with and without surrounding shell members are seen in U.S. Pat. Nos. 4,887,788 including a core of incompressible dampening material; 5,014,474 including alternating layers of elastomeric load bearing pads, one type accommodating sliding motion between a structure and its foundation; 4,910,930 including spaced steel shim plates with rubber layers sandwiched therebetween and vulcanized in situ with the shim plates; and 4,899,323 including laminated rigid hard plates and soft plates of a viscoelastic material having certain physical properties. The latter patent includes rubbers or thermoplastic resin with or without a powder or fiber filler.
By mounting a structure on a system of such horizontally soft support, the fundamental frequency of the isolated system can be made to be much lower than the fixed-base natural frequency of the structure and, consequently, the predominant frequency of the earthquake ground motion. By so doing, the bearings deflect the earthquake motion from entering the structure. Thus, earthquake protection for the structure can now be provided not by the expensive, brute-force method of building an exceedingly strong structure, but by the economic and elegant alternative of using base isolation.
It is thus desirous that a seismic isolation bearing be constructed which minimizes or essentially prevents interstory drift and floor accelerations in the multi-story building, provides stability for the design displacement due to the earthquake, increases resistance with increasing displacement and which has properties that do not degrade under repeated cyclic loading.